Removal of sulfur from exhaust gases is an important environmental task facing many industrial manufacturers throughout the developed world. Many processes for the removal of such sulfur involve the purification of exhaust gases by the removal first, by separation, of the sulfur which is, in the gas, in a hydrogen sulfide form.
One example of a manufacturing process which produces sulfur in exhaust gas is the preparation of titanium dioxide pigments. Other manufacturing industries producing sulfur in exhaust gases include, for example, electricity generation, paper and pulp making, automotive manufacture, various industries utilizing coal as a heating or other type fuel and pharmaceutical manufacture. Many other manufacturing processes in various industries from chemical manufacturers to petroleum companies face similar problems of sulfur removal from exhaust gases.
It has been considered most practical to carry out the separation of the sulfur before the disposal to the atmosphere, or in some cases burning of the exhaust gas, because the quantity of gas to be purified is thus considerably smaller. The sulfur removal from the gas can comprise two or more steps. Normally the hydrolysis of carbonyl sulfide occurs followed by the oxidation of the resulting hydrogen sulfide.
The removal of hydrogen sulfide from gaseous streams, such as the waste gases liberated in the course of various industrial chemical processes, for example, in the pulping of wood, has become increasingly important in combating atmospheric pollution. Such waste gases not only have an offensive odor, but they may also cause damage to vegetation, painted surfaces, and wild life, besides constituting a health hazard to humans. The authorities in many countries including the United States and Western Europe have increasingly imposed lower and lower tolerances on the content of such gases vented to the atmosphere, and it is now imperative in many localities to remove virtually all of the hydrogen sulfide, under the penalty of an absolute ban on continuing operation of the plant.
A number of U.S. patents describe methods of removing hydrogen sulfide from a gaseous fluid stream. Several patents in the name of Ari Technology, and its predecessor companies, discuss various removal methods. See for example, U.S. Pat. Nos. 4,622,212, 4,218,342 and 4,014,983. Ari Technology sells a commercial process for such removal under the tradename "LO-CAT Process".
Washings with oxidizing solutions have been considered for separating hydrogen sulfide from exhaust gases. In such process, a very finely divided sulfur precipitates into a solution which is an alkaline suspension of sulfur. When filtered, the resultant product, called a filter cake, proves difficult to handle, most often on account of its thixotropy. This has lead companies to utilize commercial processes which recover the sulfur in a molten, and not a solid form without a filter cake. This has proved costly and has required expensive complicated equipment.
Such costly prior art recovery processes have generally heated sulfur-containing suspensions under pressure to about 140.degree. C. The sulfur melts thereby forming a liquid phase, which is then drawn off. U.S. Pat. No. 4,730,369 describes a method and apparatus for recovering sulfur from an aqueous slurry. The slurry is heated sufficently to melt the sulfur and two different separation zones, in an extremely complicated apparatus, separate and transport the sulfur and the aqueous liquid. Methods prior to this patent for liquefying sulfur in an aqueous slurry typically involved passing the slurry through shell-and-tube heat exchangers having horizontal tubes, and conveying the heated effluent to a separator in which phase separtion would occur. The process often involved supernatant aqueous catalytic reagents being recycled to the process, and the lower liquid sulfur phase being transferred by appropriate means to storage. In such operations if the concentration of solid sulfur in the suspension exceeded relatively low values, e.g., 5-10%, the solid sulfur had a tendency to plug the tubes of the exchanger. The efficiency of separation in the separating vessel was poor, since the relatively high velocity of the aqueous phase, passing vertically through the separator captured small particles of sulfur and thus lead to operating difficulties in other sections of the process. This process is expensive with regard to energy and for smaller installations is uneconomical because of high capital cost. For further production of usable powdery sulfur, the liquid sulfur product would then have to be solidified, broken-up and ground. In U.S. Pat. No. 4,304,570, elemental sulfur obtained from the scrubbing of exhaust gases is suspended in a solution and is preheated with steam to a temperature above the melting point of the sulfur with complicated downstream apparatus then handling the molten sulfur. As recently as 1989, U.S. Pat. No. 4,876,079 to Linde Aktiengellschaft described a process for separating sulfur from a sulfur/alkaline solution by first heating the solution to a temperature above the melting temperature of the sulfur changing the sulfur from a solid state to a liquid state.
In view of the cost and inconvenience of these molton sulfur processes, commercial manufacturing engineers have long sought without success a simple "solid" process to substantially improve the filterability of dispersed sulfur suspensions accruing from an exhaust gas wash in which a non-thixotropic filter cake with high solids content was produced.